Health.

That, more than anything, will be the key ingredient in determining whether the Boston Red Sox will be able to maintain their cushy 10-game lead in the American League East and waltz into the playoffs.

But even before the first pitch of the post-All-Star break has been thrown — the Sox resume the schedule tonight at home against the Toronto Blue Jays — already there is a health issue that could blossom into a major obstacle to Boston’s division-title aspirations and quest for a postseason berth after a one-year absence from the playoffs.

That, of course, would be David Ortiz’s right knee.

It was reported by the Boston Herald at the All-Star Game in San Francisco that Ortiz, who has been known to have problems with his right quadriceps and right hamstring, also has an issue with his right knee, serious enough that the word “surgery” has been mentioned.

It’s not necessarily a major injury. And it’s possible that whatever specific medical problem it happens to be that Ortiz will be able to play through the discomfort and hold off surgery until after the season.

But the information sheds better light on the fact that at the break Ortiz had only 14 home runs, a paltry total for the Red Sox designated hitter, who set a franchise record with 54 homers a year ago.

Now it makes more sense as to why the long balls haven’t been jumping off Ortiz’s bat. It hasn’t been about the pitching. It hasn’t been about the umpires’ wide strike zones. It hasn’t been about cold weather and unfavorable wind patterns.

The bottom line is that if Ortiz has pain when he tries to shift his weight from his back leg to his front (right) leg, a major component in driving a baseball, it’s extremely difficult for him to generate any more power in the second half than he showed in the first half.

And that could be a big worry for the Red Sox as they open the second half with a four-game set with the Blue Jays, who are tied with the New York Yankees for second place in the A.L. East.

The series with the Jays opens an 11-game home stand, the longest of the season for the Sox. Boston, which boasts a 28-14 record at Fenway, the third-best home record in the A.L., will be hoping to get the second half off to a fast start at home because after this stretch, 26 of the next 35 games will be played on the road, including a third West Coast trip and three games in Yankee Stadium (Aug. 28-30).

Health, of course, is promised to no one. The Red Sox need only look back at their calamitous August and September of last season, when players dropped like flies, one after the other in a seemingly never-ending situation of bad timing and bad luck. Or they could check out the Jays and the Yankees over the first month or two of this season.

As they look ahead to the second half, the Red Sox face several questions, not unlike the questions facing virtually every general manager as the determination is made whether his particular team will be a “buyer” or a “seller” at the July 31 trading deadline.

Will the Sox’ roster change at the trading deadline? Should it? And if so, what direction should general manager Theo Epstein pursue? Another starting pitcher? Bullpen help? Or a big bat either off the bench or somewhere in the starting lineup, maybe someone to platoon $70-million man J.D. Drew in right field?

Those questions, meanwhile, lead to other questions, more in-house types of evaluations that could influence July 31 activity.

Do the Sox have the pitching help they may need for the stretch run and beyond in the farm system? Do they have enough talent in the farm system — talent they’re willing to ship off — to satisfy the “sellers” in the market and obtain what Epstein feels is necessary.

As the Sox head into the trading season, they seem to have a few “untouchables” in the minor-league system, notably pitchers Jon Lester and Clay Buchholz and center fielder Jacoby Ellsbury.

Lester could be the stretch-run infusion of talent to the rotation that the Sox may need and would come, obviously, without having to give anybody up. The left-hander has big-league experience, and is a touted prospect, but, coming back from his bout with cancer that was discovered about 11 months ago, Lester still hasn’t been as dominant as the Red Sox would like to see in Pawtucket.

Buchholz, a right-hander, is Pawtucket-bound from Portland and could arrive as a power arm for the bullpen, á la Jonathan Papelbon in 2005. Again, there would be no cost involved in his promotion, and Boston’s starting rotation could receive another “in-house” boost if and when 40-year-old right-hander Curt Schilling (shoulder tendonitis) is able to come off the disabled list.

There is little doubt that, in a long season, it never hurts to add fresh arms to the pitching staff. There will be some veterans who are eligible for free agency after the season, or big-bucks pitchers that out-of-the-running teams are looking to move for prospects, but at what cost to the Sox?

Craig Hansen, a first-round pick in 2005 who was untouchable only a year ago, has struggled this year in Pawtucket? Wily Mo Pena? Kason Gabbard? Craig Breslow? David Murphy? Brandon Moss? Deeper into the system?

Ellsbury, meanwhile, was impressive in his cup of coffee with the big club a few weeks ago, so if and when the Red Sox feel the need for a fourth outfielder and wheels off the bench, Boston knows it needs only dip down to Pawtucket for that type of reinforcement.

Offense, though, is another story. The Red Sox have been struggling mightily to score runs the last six weeks or so. There has been plenty of talk that veteran hitters with track records find their levels by the end of the season.

But will his injury keep Ortiz from coming close? What’s up with Manny Ramirez and his measly 11 homers? Should the club expect 20-25 homers from him in the second half? Will Lugo climb over .200 and keep rising? How about a little production from Drew? Can the Sox count on that happening? Can Kevin Youkilis and rookie Dustin Pedroia maintain their first-half success?

On the flip side, Mike Lowell is a career .273 hitter who earned an All-Star berth based on his .300 batting average and 63 RBI. But last year he was batting .307 with 46 RBI before the break and only .257 with 34 RBI in the second half. Will that trend hold up?

If the Sox are only going to tinker with the offense, the only place there is to add a bat would be in right field or off the bench, preferably in the same package. Who else on this team might you replace and be able to find another potent bat to fill the position? The only real option would be to platoon Drew with a right-handed hitter.

If Epstein wants to swing a more major deal, of course, the holes created by the departures of players have to be filled by the incoming ones.

At this point, though, the Red Sox are dealing from strength. Boston has a 10-game lead, despite offensive woes, because of consistently solid starting pitching and bullpen help to hang onto victories despite spotty contributions at the plate.

That’s a nice position to be in as the second half dawns. But it doesn’t necessarily mean that this team can breeze into the postseason without any second-half alterations. And the players’ health, naturally, will be the major unknown that could affect and sink Epstein’s entire master plan, whatever it may be.

skrasner@projo.com